1. Field of the Invention
The present invention relates to an MPEG data recorder, and more particularly, relates to an MPEG data recorder, which selects a recording mode in accordance with a data-rate and records the MPEG data transmitted in real time through a digital transmission line such as IEEE 1394.
2. Description of the Related Art
It is possible to record digital data on a magnetic medium, which is one of recording media, by using a D-VHS (Digital-VHS) recorder. In this case, since the amount of consumed tape increases as the data rate of data to be recorded becomes higher, a plurality of recording modes for the different conveying speeds of the tape are defined in the D-VHS standard. Accordingly, it is possible to select a proper tape speed to improve the efficiency of usage (recording) of magnetic tape, by selecting a recording mode in accordance with the data rate.
As the recording modes for D-VHS, HS mode of 28.2 Mbps (bit/sec), STD mode of 14.1 Mbps, LS3 mode of 4.7 Mbps, LS5 mode of 2.8 Mbps, etc., have been defined. For example, when MPEG transport stream (MPEG-TS) data are to be recorded with a D-VHS recorder, if its data rate of the MPEG-TS to be recorded is 4 Mbps, it is preferable to record in LS3 mode, because it would be impossible to record in LS5 mode, while it would consume excessive tapes in HS mode or STD mode.
When MPEG-TS data are to be recorded in the D-VHS recorder, the maximum data rate of the MPEG-TS data is determined for selecting a recording mode. This maximum data rate can be determined by analyzing a descriptor, which has been multiplexed to an MPEG-TS.
FIG. 12 is a diagram of an example of the structure of an MPEG data recorder, and showing an outline of the structure of a D-VHS recorder, which records the MPEG-TS data transmitted in real time through an IEEE 1394. In the figure, numeral 1 denotes an IEEE 1394 interface unit; 2 denotes a D-VHS data processor; 3 denotes an ECC processor; 4 denotes a buffer memory for the ECC process; 5 denotes a record signal processor; 6 denotes a record amplifier; 7 denotes a recording head; 8 denotes a magnetic tape; 9 denotes a motor driver; 10 denotes an MPEG system layer decoder; 11 denotes a decode memory; and 12 denotes a recording mode controller, respectively.
The IEEE 1394 interface unit 1, which is connected with an IEEE 1394 link, transmits and receives asynchronous data such as control commands and isochronous data such as MPEG-TS. The interface unit 1, on receiving MPEG-TS data via the IEEE 1394 link, supplies the MPEG-TS data to the D-VHS data processor 2 and the MPEG system layer decoder 10.
The format of MPEG-TS data outputted from the interface unit 1 is converted in the D-VHS data processor 2, and an error correction code is added to the data in the ECC processor 3. Then, MPEG-TS data are subjected to a predetermined signal processing in the record signal processor 5, amplified by the record amplifier 6, and recorded on the magnetic tape 8 by the recording head 7. The conveying speed of the magnetic tape 8 is controlled by the motor driver 9.
On the other hand, MPEG-TS data outputted from the interface unit 1 is analyzed in the MPEG system layer decoder 10, in which program specific information (PSI) and service information (SI) are extracted. The decode memory 11 is a memory for temporarily expanding the data when the MPEG system layer decoder 10 analyzes the PSI and SI. The recording mode controller 12 detects the bit rate of MPEG-TS to select a recording mode, based on the PSI and SI analyzed by the decoder 10. Since, in the D-VHS standard, the recording modes are classified by the conveying speed of the magnetic tape 8, the motor driver 9 controls variably the conveying speed of the magnetic tape 8 to adapt for the selected recording mode.
When MPEG-TS data are those which have been inputted from an IEEE 1394 link, the data rate shows various values. For example, in the case of BS digital broadcast, it gives about 26 Mbps in HDTV program, and it gives about 6 Mbps in SDTV program. Furthermore, in the case of CS digital broadcast, it ranges from about 4 Mbps to about 6 Mbps, or it could be up to about 20 Mbps when MPEG-TS data carried on a broadcasting wave are to be recorded directly. These values can be obtained directly or indirectly by detecting and analyzing the information, called a descriptor, contained within the MPEG-TS.
MPEG-TS data outputted from the IEEE 1394 interface unit 1 are inputted to the MPEG system layer decoder 10, expanded into the decode memory 11, and subjected to a demultiplexing process of multiplexed information (Demux process). In this process, the maximum data rate information of the transmitted MPEG-TS data can be obtained, by analyzing the descriptor named a maximum bitrate descriptor, which has been inserted, e.g., into the PMT (program map table) multiplexed to the MPEG-TS data. The data rate information, thus obtained by the MPEG system layer decoder 10, are supplied to the recording mode controller 12. The recording mode controller 12 selects the most proper recording mode, in which MPEG-TS data are to be recorded, based on the obtained data rate information, and the motor driver 9 adjusts the tape-conveying speed in accordance with the recording mode.
In such an MPEG data recorder as described heretofore, the recording mode is selected by detecting the data rate of MPEG-TS data with using the system layer decoder. Namely, the MPEG-TS system layer decoder is required for selecting a recording mode. However, in the case an MPEG data recorder having no such a decoder employed, as in a low-price D-VHS device, there is a problem that the data rate of MPEG-TS data cannot be detected and thereby the selection of recording mode would be impossible.
In such a recorder having no system layer decoder, since the recording mode is selected independently of the data rate of MPEG data, it is necessary to record all the MPEG data in a recording mode of the highest data rate, e.g., HS mode of 28.2 Mbps. If the actual data rate of MPEG data is low, almost all of the D-VHS formatted data, which have been recorded on a magnetic tape, are redundant (stuffing) data, and thereby it leads to a problem of very low recording efficiency.